1. Field of the Invention
The present invention relates to a device for varying the energy of a particle beam extracted from a particle accelerator.
The present invention also relates to the use of said device.
2. State of the Art
Certain applications involving the use of beams of charged particles also require the energy of these particles to be rapidly varied.
To do this, one solution consists in using an accelerator capable of producing, intrinsically, an extracted particle beam whose energy is variable. In this regard, it may be proposed to use an accelerator such as a synchrotron capable of producing within this accelerator itself a particle beam, the energy of which is variable. Nevertheless, this type of accelerator is relatively complex to produce, and is accordingly more expensive and less reliable than particle accelerators which produce beams of fixed energy such as cyclotrons.
As a result, it has been proposed to equip such fixed-energy accelerators with a device whose function is to modify the energy characteristics of the beam, and to do so over the trajectory of said beam extracted from the accelerator. These devices are based on the well-known principle according to which any particle passing through a block of material undergoes a decrease in its energy by an amount which is, for particles of a given type, a function of the intrinsic characteristics of the material passed through and its thickness.
Nevertheless, the main drawback of such devices, which are also known as energy degraders, lies in the fact that the block of material deteriorates the energy resolution of the degraded beam. This is due to a phenomenon which is also known as xe2x80x9cstragglingxe2x80x9d, which generates a static energy variation of more or less 1.5%. By proposing an entry face and an exit face that are parallel within the energy degrader, this phenomenon tends to be reduced.
In addition, it is observed that the optical characteristics of the beam passing through the energy degrader are also altered. In particular, a parallel incident beam becomes divergent when leaving the degrader because of the multiple scattering within the degrader. These drawbacks (increase in divergence and in energy dispersion) may lead to a situation in which the emittance of the beam is too high to meet the entry emittance constraints set by the optical elements of the beam which are located downstream along the beam transport line.
In order to solve these problems, it has also been proposed to use an analysis magnet placed after the degrader device, which is intended to accept only the energy desired for a predetermined resolution, with the aid of slits and collimators provided to improve the optical characteristics of the degraded beam. Nevertheless, by using such elements, it is observed that the intensity of the beam is further reduced, also causing a large activation of the various elements.
The document xe2x80x9cThree-dimensional Beam Scanning for Proton Therapyxe2x80x9d from Kanai et al. published in Nuclear Instruments and Methods in Physic Research (Sep. 1, 1983), The Netherlands, Vol. 214, No. 23, pp. 491-496 discloses the use of a synchrotron which produces a beam of protons controlled by means of scanning magnets, which is then directed towards an energy degrader having as function to modify the energy characteristics of the proton beam. This degrader substantially consists of a block of material whose thickness is discretely variable. Nevertheless, this application does not propose to perform a continuous variation of the energy of the beam extracted from a particle accelerator, and in particular a fixed-energy particle accelerator.
The present invention aims to provide a device which would make it possible to vary the energy of the beam extracted from a particle accelerator, in particular from a fixed-energy particle accelerator.
More particularly, the present invention aims to provide a device which would make it possible to vary almost continuously the energy of a beam extracted from a particle accelerator.
The present invention relates to a process and a device for varying the energy of a particle beam extracted from a fixed-energy particle accelerator. With this aim, an energy degrader is inserted in the path of the particle beam extracted from the accelerator, this degrader substantially consisting of a block of material, the thickness of which is discretely variable by steps. The thickness is defined as the distance between the entry face and the exit face on the block of material.
The energy difference between the steps is variable and is determined such that the variation in the intensity of the beam reaches, at the limit between two consecutive steps, a maximum of 15% and typically 10% of the maximum intensity obtained at the exit of each of the two successive steps under consideration. This makes it possible to obtain a continuous variation of the energy despite the fact that the thickness varies discretely. Indeed, this is due to the combination of the way of calculating the energy difference between the steps with the association of an analysis element.
According to one preferred embodiment, this degrader is positioned at the point at which there is a narrowing (xe2x80x9cwaistxe2x80x9d) of the beam envelope. In addition, the curvature of the entry and exit faces of the degrader, defined by the height of the discrete levels or steps, is designed such that the xe2x80x9cwaistxe2x80x9d is always for each step or level at the ideal position relative to the entry and exit faces without requiring the modification of the beam transport control parameters, and in particular the position of the xe2x80x9cwaistxe2x80x9d, from one step to the next.
This advantageously allows to keep the characteristics in energy dispersion and the optical qualities of the beam.
The energy degrader preferably has steps or levels of variable width, the width of a step being defined as the distance between two successive steps. This width should be adjusted such that it is slightly larger than the diameter of the beam entering or exiting the degrader, which means that the width of said steps or levels of large thickness will be greater than the width of said steps or levels of small thickness.
The material of which the energy degrader is made should have a high density and a low atomic mass. Examples may be diamond, aggregated diamond powder or graphite.
An analysis magnet may also conventionally be combined with this energy degrader.